structure and function of the eye Flashcards
what are the 3 types of tear production
basal tears
reflex tears - response to irritation - CN V1 afferent from cornea, efferent is PNS - NT is ACh
crying - emotional
describe the larcimal system *
biggest comp of teasrs produced by the lacrimal gland
drains through the 2 puncta, opening on the medial lid margin
flow through the superior and inferior canaliculi
gather in the tear sac
exit through tear duct into the nose
describe the tear film *
maintains smooth cornea-air surface, oxygen supply to cornea, rremoval of debris, bacteriocide
3 layers:
superficial oily layer to reduce tear film evaporation (produced by a row of meibomian glands along the lid margins)
aquous tear film - tear gland
mucinous layer on the corneal surface to maintain surface wetting from goblet cells - cushion and protect eye from infection.
what is teh conjunctiva *
thin transparant tissue that covers the outer surface of the eye
nourished by tiny bv nearly invisible to the naked eye
where is the cornea *
begins at the outer edge of the cornea, covers the visible part of the eye, lines the isnide of the eyelids
coats the sclera
fold in conjuctiva sack - protect structures around the eye
when do the blood vessels supplying the conjunctiva become visible *
conjuctavitis and conjunctival hyperaemia
uveitis - inflammation of the uvea
what is the vitreous *
structure in the eye that keeps it in its shape
what is the size of the eye *
24mm diameter
what are the 3 layers of the coat of the eye *
the sclera - hard and opaque
choroid - pigmented and vascular
retina - neurosensory tissue - transparant - light pass through and hit phototreceptors
describe the sclera *
commonly known as the white of the eye
tough opaque tissue that serves as the eye’s protective outer coat
high water content
coat for 2/3 of eye - then contuous with sclera
describe the cornea *
transparent
dome shaped window covering the front of the eye
powerful refracting surface, providing 2/3 of eyes focussing power - conves, higher refractive period than air
low water content
front most part of teh anterior segment
continious with the scleral layer
physical barrier
infection barrier
what is the structure of the cornea *
5 layers:
epithelium - external wall
Bowman’s mem - basement mem of epi
stroma - regularity contributes towards its transparancy, corneal nerve endings provide sensation and nutrients for healthy tissue, no bv in normal cornea
descemet’s mem - basement membrane for endotheliym
endothelium - pumps fluid out of cornea - preventing oedema - only 1 layer, no regeneration piowe , density decrease with age, dysfunction = oedema and cloudiness
what happens if you hydrate the cornea *
goes white
what is the uvea *
vascular coat of teh eye
between the sclera and the retina
composed of - iris, ciliary body and chroid
these 3 portions are closely connected - disease of 1 part affect the others
describe anterior intermediate and posterior uveitis *
anterior - normally transparant but in uveitis can see inflammatory cells - if dilate the pupil and look behind - it is still transparant - treat topically
intermediate and posterior - cells in the vitreous too - treat systemically
describe the choroid *
between the retina and sclera
coats the rretina
composed of layers of BV that nourish back of eye
describe teh blood supply of the retina *
2 blood supplies
from choroid - supply outer part of the retina
inner retinal arteries from the central retinal vein that gives off the central retinal artery - noruishh the inside of retina
what is teh retina *
very thin layer of tissue
lines the inner part of the eye
function of the retina *
capture the light rays that enter the eye
impulses are sent to the brain for processing via the optic nerve
describe the optic nerve *
transmit electrical impulses from the retina to the brain
connects to back of eye near macula
visible portion is called the optic disk
describe how you can see the nerves in the eye
using polarised/red light
optical coherence tomography - see thickness of the nerves
what is the importance of looking at the bv at the back of the eye
abnormalities here reflect abnormalities in the rest of the bv
what is the macula *
small, highly sensitive part of retina - responisible for detailed central vision
describe the anatomy of the fovea *
located roughly in centre of the retina
avascular
fovea at teh centre
role of the macula *
appreciate detail
perform tasks that require central vision eg reading
what is the 1st cause of irrevesible blindness *
glaucoma - silent until late stages
what are the landmarks in the eye *
the optic disk
the macula - avascular
bv - clsoer to optic disk = thicker
what are the anterior and posterior segments of the eye *
anterior segment - ocular structure, anterior to the lense
posterior - ocular structure posterior to the lens
describe the anterior chamber *
between the cornea and the lense
filled with clear aq fluid
supplies nutrients
describe the posterior chamber *
between the posterior aspect of the iris and the zonula
role of ciliary body *
production of aq humour
path of the aq humour *
produced by the ciliary epi
pass through post chamber
through pupil
fill the anterior chamber
drain into angle by the trabecular meshwork - schlemm’s canal - specialised vein draining 80% fluid, rest is drianed passively by uveal scleral outflow
function of the aq humour *
supplies glucose to cornea
normal intraocular pressure *
12-24mmHg
definition of glaucoma *
optic neuropathy with characteristic structural damage to the optic nerve
associated with progressive retinal ganglion cell death, loss of nerve fibres and visual field loss
describe glaucoma *
risk factor is raised intraocular pressure - not essential, can have raised pressure and not glaucoma and vice verca
age, family history and ethnicity also risk factors
have retinal ganglion cell death and enlarged optic disk cupping
cause visual field loss and blindness
treatment of raised intraocular pressure *
medical
surgery - increase outflow or reduce production
parasurgery - lasers - create drainage hole on iris
wat are the 2 types of glaucoma, describe them *
primary open angle glaucoma - commonest - trabecular meshwork dysfunction
closed angle gaucoma - acute/chronic - increased pressure pushes the iris/lens complex forward - block the trabecular meshwork - risk factors: small eye, narrow angle at trabecular meshwork - may present with sudden painful redeye and acute drop in vision
wat would you see with glaucoma *
missing details - brain fills in areas tjat you cant see with things that are around it
what is the visual difference between arterioles and venules in the eye
venules - thicker and darker
why is the optic nerve a blind spot *
no photoreceptors here - dont normally notice it becausse the brain fills in the gap
if sudden change here - brain cant fill gap - see black
describe photoreception in the fovea *
highest concentration of cones - able to percieve detail
low conc of rods - which are more sensitive to light than cones- why objects in periphery look brighter
descrive central vision *
detail and colour vision
reading and facial recognition
assessed by the visual acuity assessment
(from fovea)
describe peripheral vision *
shape movement and night vision
navigation
assessed by visual field assessment
loss = unable to navigate in environment - may need white stick even witth perfect central vision (vision acuity)
describe the retinal structure *
outer layer (1st order neuron) - detection of liught
middle layer (2nd order) - bipolar cells - local signal processing to improve contrast sensitivity and redulate sensitivity
inner layer (3rd order) - retinal ganglion cells - transmission of ssignal from eye to brain
describe the orientation of photoreceptors in the retina *
face away from the pupil
light bounce on retinal pigment epi then hit the photoreceptors - some hit teh straight aprt and some hit the curved part (rretina is curved) - poroprtion of retina that is hit tells us position is space
pupil black because light isnt reflected back out becuase rretina epi is dark so absorbs most of light - only see ‘red reflex’ when shine light directly into eye
describe rods *
they have a longer outersegment with phhottosensitive pigment
100x more sensitive to light than cones are
slow response to light
scotopic vision (night)
120 million rods
describe cones *
less sensitive to light
faster response than rods
photopic visiion (day and colour)
6 million
describe the photoreceptor distribution relative to fovea *
no receptors in the blind spot - 10-20 degrees
cones peak at 0 degrees
rods peak at 20-40 degrees
describ ethe frequency spectrum *
S cones - blue light 400-450 nm (WL)
M cones - green - 530-560
L cones - red - 560-600
rods - 500nm
UV - less than 400nm
blue laser - see front of retina, greenn- intermediate, red- deep
what is the most common colour deficiency *
deuteranomaly - confusion of red and green
also called daltonism
what is acromatopsia *
total colour blindness - just see grey etc
what is the ishihara test *
colour perception test
ishihara plates test for red-green deficincies only
plates with circles occuring randomly in size
normal will see teh correct 2 diugit number
abnormal - no number/wrong
what is special about number 25 of the ishihara test *
everyone can see - even with acromatopsia - look different shhades of grey
what is dark adaption *
increase in light sensitivity in dark
photoreceptors are firing all the time - so when see bright light, they continue firing afterwards - so keep seeing the bright image - so need to dark adapt for a length of time
biphasic process - cone adaption for 7minutes, rod adaption for 30minutes
describe light adaption *
adaption from dark to L occurs over 5 mins
bleaching of photopigments
neuroadaption
inhibition of cone rod function
wat is refraction *
as light goes from 1 medium to another, the velocity changes
as light goes from 1 medium to another, its path changes
what is an index of refraction
= speed light in vacuum/speed light in medium
always >/=1
what does light do when it hits a new medium *
some light reflects off teh boundayr
some refracts
angle of incidence = angle of reflection
what does a converging lense do *
takes light rays and brings them to a point
what does a concave lense do *
takes ligt rays and spread them out
virtual focal point behind the lense
what is emmetropia *
there is adequate correlation between axial length and refractive power - parallel light rays fall on the retina - no accomodation
what is ametropia *
mismatc between axial length and retractive power
parallel light rays dont fall on the retina
eg
nearsightedness - myopia
farsightedness - hyperopia
astigmatism
presbyopia
what is myopia *
parallel rays converge at a point anterior to the retina
eitiology not clear - genetic factor
causes - excessive long globe (axial myopia) more common
excessive refractive powerr (refractive myopia) - cornea/lense are too convex
from clsoe objects light rays come from an angle - so focal point is further away = can read
symptoms of myopia *
blurred distance vision
squint too improve visual acuity when gazing into distance - trying to only get light ocming straight so that it doesnt get refracted
treatments for myopia *
concave lense - make rays diiverge = focal point on retina
contact lens
refractive surgery - flattern the cornea = reduced refraction
remove lens and hence refrcctive power - not done anymore
what is hyperopia *
parallel rays converge at a focal point posterior to the retina
eitiology not clear - inherited
causes: excessive short globe (axial hyperopia) - common, insufficient refractive power (refractive hyperpia) - lens flatter, can accomodate within acertain degree
symptoms of hyperopia *
visual acuity tends to blur early - either cant read fine print, or is fine and then suddemnly and intermittently blurry - worse when lighting weak, tired, print is weak
asthenopic problems - eyepain, headache in frontal region, blurring sensation in eye, blepharoconjunctavitis
amblyopia - uncorrected hyperopia - ?
treatment for hyperopia *
convex glasses
contact lensess
intraoptic therapy
what is astgmatism *
parallel rays come to focus in 2 focal lines rater than 1
eitiology - herefditory
casue - cornea not spherical - refract differently along one meridian compared to a perpendicular meridian = >2 focal points
punctifom object present as 2 sharply defined lines
2nd focal point wont fall on the retina - confusing the images taht you see `
symptoms of astigmatism *
asthenic problems - headache, eye pain
blurred vision
distortion of vision
head tiliting and turning
confusion of images
treatment of astigmatism *
regular atigmatism - cyliinder lenses with or without spherical lenses (convex/concave) - light passing throug centre will not be refracted, light passing through edge hit concave - bend - reach focal point - roate it to correct just 1 area
irregular astigmatism - rigid contact lenses, surgery
what is the near response triad *
for adaption to near vision
- pupillary miosis (sphincter pupillae constrict) to increase the depth of field
- convergence (medial recti from both eyes) - to align both eyes to a near object
- accomodation (circular ciliary muscle) increase refractive power of lens for neaar vision
what is presbyopia *
naturally occuring loss of accomodation - focus on near
onset from 40yrs
distant vision in tact
corrected by reading glasses (convex lenses) to increase the refractive power of the eye
treatment for presbyopia *
convex lenses for near vision - reading glasses, bifocal, trifocal, progrressive power glasses
what are the different types of optical correction *
specticals - monofocals (spherical lenses, cylindrical) multifocal
contacts - higher quality of optical image and less affect on the size of the retinal image than specticals, indication - aesthetic, athletic, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
disadvantages and complications of contact lenses *
careful daily cleaning, disinfection, expense
infectious keratitis, giant papillary conjunctavitis, corneal vascularisation, severe chronic conjuctavitis
describe intraocular lenses *
replacement of cataract crystaline lense
give best correction for aphakia, avoid significant magnification and distortion by specticals
describe surgical correction *
keratorrefractive surgery - RK, AK< PRK, LASIK, ICR, thermokeratoplasty
intraocular surgery - clear lens extraction, phakic intraocular lens implantation - but lens in front of the natural lens
laser surgery - remove he epithelium of the cornea, make a flap, use laers to adjust the cornea, return flap
phacoemulsification - remove the natural lens and implant of artifial folded lens
describe the macula and fovea *
macula lutea is the pigmented region at the centre of the retina - 6mm diameter
fovea - pit at centre - no overlapping ganglion cell layer- highest conc of photoreceptors for fine vision
how can you clinically asssess the eye
opyical coherence tomography - send diff wl into eye - pick up differnet structures - can reconstruct the eye
see fluid, attachments and path
describe the process of accomodation *
contraction of the circular ciliary muscle inside the ciliary body
= relaxed zonules betweenciliary body attachment and lens capsule attachment
lens returns to natural convex shape due to elasticity- increasing the refractive power of the lens
this is mediated by CN 3
